In using a conventional hydraulic jack, despite of dummy load or loading condition, the handle must move repeated many times so as to drive the oil cylinder to work. However, since in dummy load or light load, it still needs to be driven repeated, much work is wasted. Therefore, the invention in U.S. Pat. No. 5,755,099, "Hydraulic Circuit System for One Touch Jack and Its Structure", discloses a jack structure, in which the jack hydraulic loop circuit includes an oil inlet circuit, an oil return circuit and an overload protecting circuit.
In the oil inlet circuit, the outer oil reservoir is communicated to the pumping oil chamber through a check valve. The pumping oil chamber is communicated to a oil guiding tube through another check valve, The oil guiding tube is embedded into the inner oil chamber in the piston rod for forming as a loop. The pumping oil chamber is communicated to the inner oil reservoir of the piston rod through a sequential valve B. The outer oil reservoir is communicated to the inner oil reservoir of the hydraulic cylinder through a check valve. When a manual pump is in dummy load or light load, the oil circuit provides hydraulic oil to the inner oil chamber of the piston rod from the pump and through the oil guiding tube.
In the oil return circuit, the inner oil reservoir is communicated to the inner oil chamber of the piston rod through a check valve, then it further passes through a release valve to be communicated to the outer oil reservoir. When oil is unloaded and then it desires to the original position, the release valve can be adjusted to a release position so that the oil return circuit is conducted.
In the overload circuit, an outer oil reservoir passes through a safety valve to be communicated to the oil chamber of a manual pump. When the pressure of the hydraulic cylinder is over a rated pressure, the safety valve will be conducted so that the overload protecting circuit will open automatically. In the aforesaid hydraulic loop system, the maximum effective oil storing amount of the pumping oil chamber of the manual pump is large than or equal to the maximum effective oil storing amount of the inner oil chamber in the piston rod so that the hydraulic jack may rapidly rise to a loading position in dummy or light load condition.
However, in above invention, for the check valves in the oil circuit and the oil channels, since when the piston rod move to a position in one touch, the oil guiding tube and the inner oil reservoir are in still condition. Thus, the inner and outer pressures of the oil guiding tube in the piston rod are unequal, the oil guiding tube must suffer from a large still load pressure. Therefore, the pressure-resistance of the oil guiding tube must be further improved, but this will increase the cost. If the pressure-resistance of the oil guiding tube is insufficient, it is possible to break out. Therefore, many considerations in safety are concerned for the prior art design.